The present invention relates generally to an optical circulator for fiber optic communication systems. More particularly, this invention relates to a low cost compact circulator.
In order to reduce the size of an optical circulator to make more compact circulators for various fiber optical applications, optical components of higher prices are often required. Thus, higher production costs are now encountered by the fiber optic industries when optical circulators are manufactured with reduced size. Specifically, higher material costs are resulted from the use of single fiber collimators with relatively large birefringent crystals and Faraday rotators. Since optical circulators are now applied broadly in fiber optic communication systems, cost reduction becomes an important subject. A compact optical circulator manufactured with lower cost can benefit the applications of the optical circulator in bi-directional communication, dense wavelength division multiplexing (WDM) and dispersion compensation.
An optical circulator is employed to transfer a light beam in a sequential order from one port to another. A circulator has at least three optical ports. These ports can be accessed in such order that when a light beam is caused to enter into an optical port, this light-beam after passing through the circulator to exit from a next optical port. For example, a first optical beam enters the circulator through the first port of an optical circulator exits from the second port; an optical beam enters the circulator from a second port exits from the third port. An optical circulator is an important passive device that has a non-reciprocal function. Optical circulators can be divided into two types, one with perfectly circular propagation structure and the other with an imperfectly circular propagation structure. The latter is also referred to as a quasi-circulator or as a circulator. In most applications, a perfectly circular propagation structure is not necessary. To apply a quasi-circulator to optical fiber systems, various kind of structure of polarization-independent optical circulators, including quasi-circulators, have been developed. However, in the fiber optical industries, a person of ordinary skill in the art is still challenged by the tasks of making a compact optical circulator with reduced size and meanwhile keeping a low production cost.
Therefore, a need still exists in the art of manufacturing and designing the fiber optic circulator to provide improved circulator configuration. Specifically, novel and improved circulator configurations utilizing components of lower cost are needed to resolve the difficulties and limitations encountered by the fiber optical industries such that compact fiber optical circulator can be manufactured at a low production cost.
It is therefore an object of the present invention to provide an improved design and configuration for manufacturing and assembling a fiber optic circulator with reduced volume and lower costs such that the aforementioned difficulties and limitations in the prior art can be overcome.
Specifically, it is an object of the present invention to provide a circulator implemented with improved configuration by employing a first set of optical components for separating the beam into to two beam components and adjusting the, polarization angles, generally referred to as state of polarization. The beam components are prepared to have state of polarization for projecting toward the exit-port at a proper angle such that the fiber in the exit port can receive. A polarization-dependent beam projection means, e.g., a Wollaston prism or a Rochon prism, is employed to project the beam components to the exit port along a direction with a projection angle compatible with the fiber at the exit port. Then a second set of optical components arranged in reciprocal order of the first set of optical components. This second set of optical components are employed for readjusting the state of polarization and for merging the beam components according to a reversed beam processing sequences into a light beam identical to the input beam and exit from the output port
Another object of the present invention is to provide a circulator with improved configuration by taking advantage of the non-reciprocal characteristic of Faraday rotator coupled with the polarization-dependent projection direction of a Wollaston or Rochon prism to symmetrically transmit a beam backward from the second port to the third port. The size of the birefringent crystals is reduced and savings in production costs are achieved.
Briefly, in a preferred embodiment, the present invention includes an optical circulator. The optical circulator includes a first eccentric dual fiber capillary having a first and a third optical port. The circulator further includes a first birefringent crystal coupled to the first eccentric dual fiber capillary for separating an input optical beam entering from the first port into an ordinary beam component (e-component) having a first polarization keeping an original optical path of the optical beam and an extraordinary beam component (e-component) having a second polarization perpendicular to the first polarization for transmitting in a separated optical path symmetrical to a centerline of the optical circulator. The circulator further includes a first pair of half wave plates (HWPs) comprising a first o-half-wave-plate and a first e-half-wave-plate coupled to the first birefringent crystal for receiving the o-component and e-component respectively for effecting angular rotations, i.e., changing the state of polarization, for aligning the first polarization of the o-component and the second polarization of the e-component into an aligned polarization. The circulator further includes a first Faraday rotator coupled to the first pair of half wave plates for rotating the aligned polarization of the o-component and the e-component into a projection-to-exit port ready polarization relative to the centerline of the circulator. The Wollaston prism coupled to the Faraday rotator for projecting the o-component and the e-component into an exit port projection direction. The circulator further includes a second Faraday rotator coupled to the Wollaston prism for rotating the o-component and e-component into a reciprocally-symmetrical polarization relative to the aligned polarization. The circulator further includes a reciprocal beam polarization and merging means coupled to the second Faraday rotator wherein the reciprocal beam polarization and merging means comprising a second pair of half wave plates identical to the first pair of half wave plates coupled to a second birefringent crystal identical to the first birefringent crystal, the second birefringent crystal in turn coupled to a eccentric single fiber capillary having a second optical port wherein the second half wave plate, the second birefringent crystal and the eccentric single fiber capillary arrange in reciprocal order relative to the first eccentric dual-fiber capillary, the first birefringent crystal and the first pair of half wave plates for reciprocally rotating polarization of the o-component and the e-component and for merging the o-component and e-component into an output optical beam identical to the input optical beam for projecting from the second optical port along the exit port projection direction disposed on the eccentric fiber capillary.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.